Austenite heat-resistant steel and ferritic heat-resistant steel have been employed as a high temperature member for power generation boilers and turbines, atomic power generation facilities, apparatuses in chemical industries, and the like because they are used for a long period of time at a high temperature under a high pressure. The ferritic heat-resistant steel is often used as a high temperature member at a temperature up to about 600° C. because it is less expensive than the austenite heat-resistant steel, has a smaller coefficient of thermal expansion, and is excellent in heat-resistant fatigue properties.
In contrast, recently, it has been examined to operate thermal power generation plants at a high temperature under a high pressure to increase an efficiency with a target of increasing a steam temperature of a steam turbine from the highest temperature of 593° C. at present to 600° C. and finally to 650° C.
In general, conventional ferritic heat-resistant steel is made by combining enhancement of precipitation achieved by an M23C6 type carbide precipitating on martensite grain boundaries and an MX type carbon-nitride dispersing and precipitating in grains with enhancement of a ferrite mother phase achieved by adding tungsten, molybdenum, cobalt, and the like, as disclosed in, for example, Japanese Patent No. 2948324. However, when the ferritic heat-resistant steel is subjected to creep at a temperature exceeding 600° C. for a long period of time exceeding 10,000 hours, the M23C6 type carbide is coarsened and the effect of enhancement of precipitation is reduced as well as a dislocation is actively recovered and a high temperature creep strength is greatly deteriorated. As disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 62-180039, a method of preventing the deterioration of the creep strength for a long period of time is to maintain the enhancement of precipitation by reducing an additive amount of carbon and precipitating a nitride that is more stable than a carbide at a high temperature and unlike to be coarsened. However, carbon is necessary to secure hardenability of the ferritic heat-resistant steel, and when carbon is simply reduced, the ferritic heat-resistant steel is not sufficiently hardened and a strength enhancing effect is reduced by a dislocation introduced in hardening. Thus, there has been not yet provided ferritic heat-resistant steel having a large creep strength for a long period of time at a high temperature exceeding 600° C.